The effect of mesencephalic central tegmental field (FTC) stimulation in barbiturate anesthetized cats on the activity of the diaphragm and the laryngeal abductors was studied. With brief stimulus trains, two effects were observed: (1) a short latency, stimulus-specific activation of these muscles and (2) phase-switching of the respiratory cycle. The characteristics of short latency driving were as follows: (1) the latency and threshold for activation of the laryngeal abductors was less than for activation of the diaphragm; (2) driving continued for the duration of the stimulus only; and (3) during expiration, the threshold for short latency driving was lowest in the early part of the phase and progressively increased throughout it. Phase-switching had these characteristics: (1) expiration-to-inspiration (E-to-I) phase-switching was obtained in all cases and, in 17% of the cases, stimulation of the same FTC site also produced I-to-E phase-switching; (2) phase-switching was a function of stimulus intensity and the time of stimulation; and (3) during expiration, phase-switching showed a threshold profile opposite to that for short latency driving. These effects could be obtained after bilateral dorsolateral pontine lesions, bilateral vagotomy, and transection at the C8 level. It was concluded that the FTC could influence breathing by two systems. One is relatively direct to respiratory motoneurons and the other engages the oscillator.